The present invention relates to a system for protecting network elements connected to a public network from access over the public network, and more specifically, to a firewall system for protecting network elements connected to the Internet.
The Internet has experienced, and will continue to experience, explosive growth. As originally designed, the Internet was to provide a means for communicating information between public institutions, particularly universities, in a semi-secure manner to facilitate the transfer of research information. However, with the development and provision of user friendly tools for accessing the Internet, such as the World Wide Web (the Web), the public at large is increasingly turning to the Internet as a source of information and as a means for communicating.
The Internet's success is based, in part, on its support of a wide variety of protocols that allows different computers and computing systems to communicate with each other. All of the Internet-compatible protocols, however, find some basis in the two original Internet protocols: TCP (Transmission Control Protocol) and IP (Internet Protocol). Internet protocols operate by breaking up a data stream into data packets. Each of data packet includes a data portion and address information. The IP is responsible for transmitting the data packets from the sender to the receiver over a most efficient route. The TCP is responsible for flow management and for ensuring that packet information is correct. None of the protocols currently supported on the Internet, however, provides a great degree of security. This factor has hindered the growth of commercial services on the Internet.
The government, in learning of the Intemet's limited transmission security capacity, has resorted to encoding secure messages using complex encryption schemes. The government abandoned consideration of the Internet for high security information, relying instead on privately operated government networks. The general public, without such concerns, has come to increasingly use the Internet. Furthermore, businesses having recognized the increasing public use of, and access to the Internet, have turned to it as a marketing mechanism through which to disseminate information about their products, services and policies.
A popular way for commercial institutions to supply information over the Internet is to establish a homepage on an Internet multi-media service known as the World Wide Web. The World Wide Web ("Web") provides a user-accessible platform that supplies information in text, audio, graphic, and video formats. Each homepage document can contain embedded references to various media. A Web user can interactively browse information by responding to entry prompts nested in a screen within a homepage. Web documents are accessed by using a TCP/IP compatible protocol called HyperText Transfer Protocol (HTTP). A user logged onto the Internet can access a "Web site" by supplying the Web site's address (e.g., "http://srmc.com"). Entry of such an address establishes a session between the user and the Web site.
Provision of a Web homepage involves establishing a user accessible file at a Web site. The Web site can be established on a computing system on the premises of the business or institution providing the homepage, or by contracting to have the homepage built and supported on the computing facilities of an Internet Service Provider (ISP). The assignee of the present application, Scientific Research Management Corporation (SRMC), is an Internet Service Provider.
Use of a company's computing system for support of a publicly accessible system, such as a Web site, can present a threat to the company's internal systems that share the same computing platform, or are connected to the publicly accessible computing platform. Furthermore, in cases where sensitive information is transmitted over the Internet to a company, such information is usually stored on the same computing system that is used for running the on-line Internet system. For instance, some businesses now publish homepage catalogs offering services and products for sale. A user can select products or services from a homepage catalog in an interactive session. After selecting the desired products or services, the homepage may present a payment screen inviting the user enter credit card information. Handling of such information over a public network such as the Internet, requires some measure of security to prevent the information from being intercepted. However, a more important consideration is maintaining the security of such information once it is received and stored in a computing system that is connected to the Internet.
Most computer crime is not in the form of data interception, but involves a network intruder, or "hacker" entering a publicly-accessible computing system and subverting security systems to access stored information. In the recent past there have been several publicized cases where hackers have stolen proprietary information from purportedly secure computers over the Internet.
In many cases where a publicly accessible application, such as a homepage, is set up on a business or institution's premises, it is grafted onto an existing computing system. The existing system also may contain other computing resources such as data bases, and/or internal network systems that are not intended for public access. Provision of a publicly accessible on-line system, such as a Web server, on such a system can provide a scenario that can be exploited by network intruders who may attempt reach systems beyond the Web server using it, or other systems bundled on the computing platform, as access paths. A company or institution may attempt to protect these surrounding systems by password protecting them, or by concealing them from the public with a system called a firewall.
Password protected systems are well known. However, a password prompt announces the presence of proprietary systems and may be an invitation for a hacker to investigate further. Because password systems are widely known, they are somewhat susceptible to hackers who have developed techniques for cracking, bypassing or subverting them. Using conventional desktop computers, hackers have been known to decipher passwords of reasonable lengths in a very short period of time. Provision of longer passwords may thwart a hacker's attempts, but at the expense of user convenience.
The term "firewall" was coined in the computer network environment to describe a system for isolating an internal network, and/or computers, from access through a public network to which the internal network or computers are attached. The purpose of a firewall is to allow network elements to be attached to, and thereby access, a public network without rendering the network elements susceptible to access from the public network. A successful firewall allows for the network elements to communicate and transact with the public network elements without rendering the network elements susceptible to attack or unauthorized inquiry over the public network. As used herein, the term "network element" can refer to network routers, computers, servers, databases, hosts, modems, or like devices that are typically associated with a computer network.
One technique used by firewalls to protect network elements is known as "packet filtering." A packet filter investigates address information contained in a data packet to determine whether the packet machine, from which the packet originated, is on a list of disallowed addresses. If the address is on the list, the packet is not allowed to pass.
One problem with packet filtering is that when unknown address information is encountered in the filtering check (i.e., the packet's address is not on the list), the packet is usually allowed to pass. This practice of allowing unknown packets to pass is based on an Internet design philosophy that promotes the ease of information transfer. Hence, most firewall systems utilizing packet filtering operate on an "allow to pass unless specifically restricted" basis. This practice is invoked with the perception that the packet will eventually be recognized and appropriately routed down stream of the packet filter. However this practice provides hackers with a means with which to bypass a packet filter.
Hackers have developed a technique known as "source based routing," "packet spoofing," or "IP spoofing" wherein address information within a fabricated packet is manipulated to bypass a packet filter. All network elements that are addressable over the Internet have an address consisting of four octets separated by periods. Each of the octets is an eight bit sequence representing a decimal number between zero and 255. A host computer on the Internet might have an IP address: 19.137.96.1. Source based routing involves a hacker inserting an address of a machine that resides "behind" a firewall into the source address field of a fictitious packet. Such a packet can usually pass through a firewall because most firewalls are transparent to messages that originate from behind the firewall, because the firewall assumes that such messages are inherently valid. To prevent this type of packet spoofing, the packet filter's list of disallowed addresses includes the addresses of elements residing behind the firewall.
Another packet spoofing technique involves setting the "session active" bit of a packet. By setting this bit in a packet, a packet filter receiving the packet assumes that a valid session has already been established, and that further packet filtering checks are not necessary, thereby allowing the packet to pass. A spoofed packet having its session active bit set can contain an "establish connection" message. Such a packet can be used to establish a session with a machine behind the firewall.
Additional packet filtering techniques involve investigations of data portions of packet to determine whether there are any suspect contents, and or investigations of suspect protocol designations. However, the drawback of these and the aforementioned packet filtering schemes is that, when used in combination, they are cumbersome. This practice impairs the speed with which packet filters do their job.
Conventional firewalls also may use an application gateway, or proxy system. These systems operate on the basis of an application, or a computing platform's operating system (OS), monitoring "ports" receiving incoming connection requests. A port is a numerically designated element contained in the overhead of a packet. A port number indicates the nature of a service associated with a packet. For example, a packet associated with the Telnet service has a port number of 23, and the HTTP service is assigned port number 80. These port number designations are merely industry suggested, a packet containing a port designation of 23 need not necessarily be associated with Telnet services. When the OS or monitoring application receives a request on a particular port, a connection is opened on that port. A program for managing the connection is then initiated, and the firewall starts a gateway application, or proxy, that validates the connection request. However, such a system is vulnerable and inefficient because of the resource intensive nature of the processes involved.
Hackers have been known to inundate a port with large numbers of slightly varying access requests in an attempt to slip a packet by an application gateway or proxy. This method of attack is known as a "denial of service attack." The typical response to such an attack is to have the OS shut down the targeted port for a period of time. This defense response is necessitated by the inefficiency of conventional port processing. The chain of processes associated with monitoring, managing, and verifying port connections is very inefficient. A denial of service attack can unduly burden system resources. Consequently, the conventional defense is to have the OS shut down the port for a period of time. This security technique prevents entry into a system through that port and restores the availability of system resources. However, it also prevents a user behind the firewall from accessing the port that has been shut down. Hence, this security measure is unacceptable.
Another problematic aspect of conventional firewall arrangements, from a security perspective, is the universal practice of combining a firewall with other packages on a same computing system. This arises in two situations. The first is where the firewall package, in and of itself, is a combination of applications. For example, Trusted Information Systems's recently released Gauntlet application is a combination Web server and firewall. The second situation is the aforementioned practice of hosting publicly accessible and/or unrelated services on a same computing platform that supports the firewall. The services sharing the platform with the firewall may include E-mail, Web servers, or even the system that the firewall is set up to protect (e.g., a database). This situation was discussed briefly above with respect to many companies' practice of grafting a firewall application onto their existing computer systems.
The provision of applications on top of, or in addition to, the firewall on a computing system provides a path through which a hacker can get behind the firewall. This is done by using the unrelated applications to attack the firewall, or to directly connect with network elements being protected by the firewall. The firewall may fail to recognize the attack because the application being exploited by the hacker is authorized to communicate through the firewall. In addition, the firewall might not be able to protect against unexpected flank attacks from shared applications because it is set up specifically to monitor requests from a designated publicly accessible application. Alternatively, the shared application may be used to completely bypass the firewall and attack, or directly connect to, a protected network element.
An example of a conventional firewall arrangement is depicted in FIG. 1. A host computer 100 communicates with a institutional computer system 106 over a public network 102 through a router 104. A router is a network element that directs a packet in accordance with address information contained in the packet. The institutional computer system 106 supports a variety of applications including a Web server 108, and an E-mail system 114. A firewall system 110 also is hosted on the institutional computer 106 to protect a port 112 that connects an internal network 116 to the institutional computer system 106. The internal network 116 may support communication between internal terminal(s) 118 and a database 120, possibly containing sensitive information. Such a firewall system 110, however, is subject to attack in many ways.
A hacker operating the host computer 100 can utilize publicly accessible applications on the institutional computer system 106, such as the Web server 108 or the E-mail system 114, to flank attack the firewall system 110 or connect to the internal network port 112. The Web server 108 or the E-mail system 114 may have authority to attach to and communicate through the firewall system 110. The hacker might be able to exploit this by routing packets through, or mimicking these network elements, in order to attach to, attack, or completely bypass the firewall system 110.
Most conventional firewalls are transparent to packets originating from behind the firewall. Hence, the hacker may insert a source address of a valid network element residing behind the firewall 110, such as the terminal 118, to a fictitious packet. Such a packet is usually able to pass through the firewall system 110. Alternatively, the hacker can set the session.sub.-- active bit in the fictitious packet to pass through the firewall 110. The packet can be configured to contain a message requesting the establishment of a session with the terminal 118. The terminal 118 typically performs no checking, and assumes that such a session request is legitimate. The terminal 118 acknowledges the request and sends a confirmation message back through the firewall system 110. The ensuing session may appear to be valid to the firewall system 110.
The hacker can also attempt to attach to the port 112. A conventional application gateway system forms a connection to the port before the firewall 110 is invoked to verify the authority of the request. If enough connection requests hit the port 112, it may be locked out for a period of time, denying service to both incoming request from the public network, and more importantly, denying access to the internal network 116 for outgoing messages. It is readily apparent that conventional firewall systems, such as the one depicted in FIG. 1, are unacceptably vulnerable in many ways.
It is readily apparent that the design and implementation of conventional firewalls has rendered them highly vulnerable to hacker attack. What is needed is a true firewall system that overcomes the foregoing disadvantages and is resistant to hacker attack.